For the plastification of thermoplastic materials, i.e. the conversion of a heterogeneous mass, e.g. of powder, granules, scraps or pieces, to a liquid, it is common to carry out a heating and mechanical action which ultimately will melt and form a homogeneous molten mass of the thermoplastic material.
For this purpose an extruder having a worm cylinder and a worm may be used, the worm effecting a mastication, kneading and shear action upon the synthetic resin which, in conjunction with heating, effects the thermoplastification.
The cylinder can be heated, in prior art arrangements of this type, by electrical resistance heaters. German open application DE-OS 35 40 024, for example, demonstrates that it is also known to provide the cylinder with passages running parallel to the axis and traversed by a heating fluid for such heating purposes. These heating passages can be closed at their ends and interconnected by manifold passages which are transverse to these longitudinal passages.
The thermoplastic synthetic resin generally is a poor thermal conductor and thus heat transfer through the wall of the cylinder or from resistance heaters disposed in heat-exchange relationship therewith requires a large contact surface between the heated surface and the injection molding mass, to effect a rapid and uniform heating. As a consequence, when rapid, brief heating of a mass of thermoplastic synthetic resin is required, relatively long, rapidly operating worms and worm cylinders are required to provide the desired contact surface. Such apparatus is relatively expensive, requires high driving powers to rotate the worm or worms, and is subject to significant wear, thereby necessitating frequent maintenance. The significant shear effect can result in uncontrolled overheating which, together with the excessive abrasive action and wear, can be detrimental to transparent plastics and can result in excessive noise in the workplace.